I. Field of the Invention
This invention relates to a process for preparing graft copolymers of water soluble monomers and polysaccharides. More specifically it relates to a process for the production of graft copolymers of polysaccharide substrates and water soluble monomers under controlled conditions such that the graft copolymer is produced efficiently and at high conversion and is readily isolated in the form of solid beads.
II. Brief Description of the Prior Art
Polymers derived from grafting water soluble monomers to carbohydrates find uses in a variety of applications including use as paper filler retention aids, adhesives, sizings, flocculants, ion exchange resins, drilling mud additives, and water treatment aids.
Method for preparing graft copolymers of polysaccharides such as starches, cellulose and gums are well known in the literature. See, for example, Block and Graft Copolymerization, Vol. 1, R. J. Ceresa, ed., John Wiley and Sons (1973). Such methods, as represented by the teachings of U.S. Pat. Nos. 3,809,644 and 3,976,552, include polymerizations in water, in water-solvent mixtures and in the dry state, and may be initiated by mechanical, chemical and irradiative techniques.
Most of the above-described methods, however, are relatively unsuited for the efficient grafting of water soluble monomers in aqueous environments where polymerization with these monomers would be most desirable, particularly when chemical initiators are employed. This inefficient grafting is mainly due to the strong tendency of these monomers to form separate non-graft polymers in the aqueous phase. For efficient grafting, it is necessary to maintain the polysaccharide substrate and the water soluble monomer(s) in very close contact i.e. at high concentration with respect to the aqueous polymerization medium. In this manner, graft polymerization becomes the dominant reaction and the tendency to form non-graft polymers is minimized.
Several factors, however, prevent the grafting of water soluble monomers to polysaccharide substrates in highly concentrated aqueous environments. With water soluble or dispersed substrates like cellulose derivatives, gums and cooked starches, the aqueous solution viscosities of even low concentrations (10-20%) of polysaccharide in water are prohibitively high and unmanageable. Thus it is not possible to graft a water soluble monomer, e.g. acrylic acid, to a soluble or dispersed polysaccharide substrate in water at, for example, 70% solids. With non-dispersed polysaccharide substrates like cellulose and uncooked starch, which can be heterogeneously suspended in water at 40-60% solids, the graft product itself once produced in water will swell and exhibit very high viscosities during polymerization. This effect usually leads to coagulation of the reaction mixture making it commercially useless.
It is therefore an object of the present invention to provide a simple and efficient process for preparing graft copolymers of polysaccharide and water soluble monomers.
It is also an object of the invention to provide such process wherein the graft polymerization may be carried out in aqueous environments at high concentrations yielding high conversion products while avoiding the viscosity related problems of prior art aqueous techniques.
It is a further objection to provide a process for the production of such graft copolymers which can be isolated directly from the reaction mixture in the form of solid beads.